Motors of this kind can be used in a variety of applications for example in automotive engineering for drives supporting brake system control, or pumps and fans. Other application areas include ventilator fans in power supply units, or spindle motors in disk drives for data processing systems, just to mention a few.
An electronically-commutated, brushless DC motor basically consists of a shaft, a rotor assembly equipped with one or more permanent magnets mounted on the shaft, and a stator assembly which incorporates a stator component and phase windings. Two bearings are mounted at an axial distance to each other on the shaft to support the rotor assembly and stator assembly relative to each other.
FIG. 1 illustrates a schematic circuit diagram of an electronic control for a three-phase DC motor. The DC motor has three phase windings (U, 12; V, 14; W, 16), schematically illustrated in FIG. 1 in star connection 10. The three windings 12, 14, 16 are connected between a positive supply busbar 18 and a negative supply busbar 20. The positive supply busbar 18 conveys the potential +UBAT, the negative supply busbar 20 conveys the potential −UBAT. The phase windings 12, 14, 16 are connected in accordance with control signals with the supply buses 18, 20 via six power switching components (T1, 22; T2, 24; T3, 26; T4, 28; T5, 30; T6, 32). The power switching components 22 to 32 are preferably power transistors. They are equipped with control connections, designated G1 to G6 in FIG. 1. The control connections correspond in particular with the power transistor gates. The application of suitable control signals to the power transistor gates energizes the phase windings 12 to 16 in the DC motor in order to control its operation. Methods for controlling a brushless electronically-commutated DC motor which are referred to are, for example, described in DE 10033561 A1 and U.S. Pat. No. 6,400,109 B1.
One differentiates between square-wave and sinusoidal motors when dealing with DC motors, particularly three-phase DC motors as used in industrial applications in automotive engineering. Square-wave energizing means that the current applied to the phase windings flows in a square pattern. The current is activated to a specified value at a given moment in time and deactivated again at another specified moment in time. Such motors usually have a trapezoidal induced voltage. FIG. 2A schematically illustrates the induced voltages of a square-wave energized or square-wave commutated motor. Switching of the phase currents should occur during operation if two induced voltages intersect, this then minimizing the torque ripple generated. Information pertaining to the respective rotor is required to switch the phase currents at the correct instance.
Detailed information about the rotor position is required if the current is not only to be activated and deactivated, but also controlled in direct relation to the rotor position. Current control i(φ) is practical, as torque formation can be influenced by suitable setting of i(φ):T(φ)=KT(φ)*i(φ)
For example, a consistent torque can be achieved on the basis of the following equations:                                                         sin              2                        ⁡                          (              φ              )                                +                                    sin              2                        ⁡                          (                              φ                -                                  π                  2                                            )                                      =                  1          ⁢                                           ⁢          or                                                                            sin              2                        ⁡                          (              φ              )                                +                                    sin              2                        ⁡                          (                              φ                -                                                      2                    ⁢                    π                                    3                                            )                                +                                    sin              2                        ⁡                          (                              φ                -                                                      4                    ⁢                    π                                    3                                            )                                      =        1            if the induced voltage Uind(φ) and, consequently, KE(φ) and/or KT(φ) and the current i(φ) have a sinusoidal flow, voltage and current are in phase and the individual motor phases (e.g. 90 electrical degrees in the case of a two-phase motor and 120 electrical degrees in the case of a three-phase motor) are shifted in relation to each other.
FIG. 2B illustrates the induced voltages of a three-phase motor. Energizing of the DC motor phases should be realized as illustrated in FIG. 2C. It consists of six sections during an electrical cycle.
The exact position of the rotor must be known to generate a sinusoidal current directly dependent on the rotor position and, consequently, the induced voltage. Decoders or resolvers are among the devices utilized in the prior art to record the rotor position. These are rotor position sensors which operate with a specific resolution NINC and can indicate the angular position of the rotor with an angular resolution of:       φ    INC    =            360      ∘              N      INC      
The current i(φ) for energizing the motor phases can be controlled in a suitable number of stages, relative to the rotor position sensor resolution.
A resolver is, in principle, similar to a transformer with a primary winding and two secondary windings. The winding ratio and polarity of the primary and secondary windings varies, depending on the angular position of the shaft. The resolver has at least two secondary windings at an angle of 90° to each other which are stationary fittings (stator). The primary winding is mounted on the resolver shaft and is termed the rotor. The stator output signals have the same frequency if the alternating voltage is induced at a constant frequency in the primary winding, but they are offset by 90°. A sinusoidal signal and co-sinusoidal signal are thus received. The peak resolver voltage varies as the shaft rotates.
The coil output signal is converted by an analog/digital converter, the two highest converter output signal bit values indicating the quadrant in which the shaft is, and the remaining bits the shaft angle at the start of each quadrant. The analog/digital converter output signal is always a binary number.
Decoders (also known as incremental decoders) generate two output signals using a glass disk (to give an example) in which uniform subdivisions are etched. There is a light source on one side of the disk and two light detectors on the other. The glass disk is mounted on the shaft, the light source and detectors being stationary components. The detectors record an interruption in the light beam caused by the disk when the disk rotates. A relative shaft rotation can be determined by counting the transitions from light to dark. Two detectors are used if the rotational direction is also to be recorded. Decoders of the type described can only record incremental shaft rotation. The absolute shaft position is recorded by a third sensor with the assistance of a so-called zero index or zero reference track.
The rotor position sensor provides data required for current control. A problem arises here, namely that the angular position of the rotor position sensor relative to the rotor is initially unknown. The prior art thus requires mechanical adjusting of the rotor position sensor relative to the rotor so that the rotor position sensor zero index coincides with a known specific rotor angular position. In particular the zero index should be adjusted relative to a certain known commutation position. Mechanical adjustment is relatively time consuming and inaccurate.